Fluid machines are conventionally known in which an expansion mechanism, an electric motor and a compression mechanism are coupled by a single rotary shaft. In such a fluid machine, the expansion mechanism generates power by expanding fluid introduced thereinto. The power generated by the expansion mechanism, together with power generated by the electric motor, is transmitted to the compression mechanism by the rotary shaft. Then, the compression mechanism is driven by the power transmitted from the expansion mechanism and the electric motor to suck fluid and compress it.
For example, Patent Document 1 discloses a fluid machine in which an expansion mechanism, an electric motor, a compression mechanism and a rotary shaft are contained in a vertically long, cylindrical casing. In the casing of the fluid machine, the expansion mechanism, the electric motor and the compression mechanism are disposed in bottom to top order and coupled to each other by the single rotary shaft. The expansion mechanism and the compression mechanism are both constituted by rotary fluid machines.
In the fluid machine in Patent Document 1, the rotary shaft has an oil feeding channel formed therein. Through the oil feeding channel in the rotary shaft, lubricating oil accumulated at the bottom of the casing is fed to the expansion mechanism disposed in an upper part of the casing. In this fluid machine, the expansion mechanism is provided with an oil return passage so that surplus lubricating oil is sent through the oil return passage back to the bottom of the casing.    Patent Document 1: Published Japanese Patent Application No. 2005-299632